Small part, real damage prevented
On rough ground the chain slaps against the chainstay. Over a season that is thousands of small impacts on the paint, and on a carbon frame it eventually goes past cosmetic — a chain dropped off the inside of the chainring can score the stay properly.
A protector takes those hits instead. It also quietens the bike noticeably, which is a pleasant side effect on a rocky descent.
Worth fitting when the bike is new, not after the paint has already gone. Clean the stay properly first so the adhesive actually holds — on a dusty frame it will lift within a week.
See also bicycle storage.
